1. Field of the Invention
This invention relates to an automatic frequency control device which is applied to radio communication apparatus operating in a burst mode, for example, an automobile telephone or the like.
2. Description of the Prior Art
FIG. 1 is a block diagram showing the component configuration of a receiving unit of a conventional radio communication apparatus. In FIG. 1, reference numeral 1 is an antenna, 2 a high frequency amplifier which amplifies a received signal input to the antenna 1, 3 a frequency converter which mixes the amplified received signal from the high frequency amplifier 2 with a local oscillation signal to convert the received signal into an intermediate frequency signal, 4 an intermediate frequency amplifier which amplifies the intermediate frequency signal, 5 a demodulator which demodulates an output signal of the intermediate frequency amplifier 4 to obtain a baseband signal, 6 a phase-locked loop which operates as a sample and hold means and which comprises a phase comparator 7, a loop filter 8, a through/hold circuit 9, and a voltage controlled oscillator 10 (hereinafter abbreviated as VCO).
Reference numeral 11 is a synchronous circuit which operates by receiving a received burst signal contained in the demodulated data;. 12 is a control circuit which receives an output signal of the synchronous circuit 11 and sets the through/hold circuit 9 in a through state in a carrier reproducing portion of the receiving burst and sets the through/hold circuit 9 in a hold state in a data portion of the received burst; 13 is a reference oscillator; 14 is a receiving local oscillator which outputs a local oscillation signal in response to an oscillation signal of the reference oscillator 13 and 15 is a transmitting carrier wave oscillator which uses the frequency of oscillation of the reference oscillator 13 as a transmitting frequency.
Next, the operation will be described for the case where a carrier reproducing portion 102 of the received burst is continuously formed as shown in FIG. 2 and alternates with a data portion 101. A signal received by the antenna 1 is amplified by the high frequency amplifier 2 and then mixed with the local oscillation signal from local oscillator 14 to be converted into an intermediate frequency signal in the frequency converter 3. The intermediate frequency signal is amplified by the intermediate frequency amplifier 4 and the demodulated data is taken out of the demodulator 5.
The synchronous circuit 11 operates upon receiving the demodulated data, and the output signal of the control circuit 12 which has received the output signal of the synchronous circuit 11 sets the through/hold circuit 9 into a through state during the carrier wave reproducing portion 102 of the receiving burst. In this state, the frequencies and phases of the output signal of the VCO 10 and the intermediate frequency amplifier 4 are compared, the error output is outputted from the phase comparator 7, and the VCO 10 is controlled via the loop filter 8 and the through/hold circuit 9 by this error output to synchronize with the output of IF amplifier 4.
Next, in a data portion 101 of the received burst, the output signal of the control circuit 12 sets the through/hold circuit 9 into a hold state, by which the output frequency and phase of the VCO 10 after control is retained, and the output signal of the VCO 10 is supplied to the demodulator 5 to be used in the demodulation of the IF signal.
Since the receiving unit of a typical conventional radio communication apparatus is arranged as described above, that is, reference oscillator 13 is not subjected to phase-locked loop correction, the output frequencies of the receiving local oscillator 14 and the transmitting carrier wave oscillator 15 are varied by output variations of the reference oscillator 13. Therefore, in particular, when the allowable error in the transmitting carrier wave frequency is very small, the reference oscillator must be highly stable, which results in high cost.
A circuit arrangement similar to that shown in FIG. 1 is described in the form of a color synchronous circuit in the TV Engineering Handbook, first edition, edited by a corporate Juridical person, TV Society, published by OHM Co., Japan, pp. 12-79.